1. Field of the Invention
The present invention relates to devices and methods for rapidly extracting time and frequency parameters from high dynamic radio signals used for direct sequence spread-spectrum communication and measurement that are corrupted with various interference signals. More particularly, the present invention system relates to extracting time and frequency parameters from the code division multiple access spread spectrum signals used in the Global Positioning System (GPS) as those signals are being either accidentally or purposely jammed.
2. Description of the Prior Art
Radio navigation systems have long been used by airplanes and ships to provide a means of electronically determining geographic position. Today, the most advanced radio navigation system is the Global Positioning System (GPS) which is maintained by the government of the United States of America. GPS radio navigation relies upon a constellation of twenty four satellites that are in six different orbits around the earth. Navigational fixes obtained through the GPS radio navigational system are based upon measurements of propagation delay times of the signals broadcast by the orbiting satellites. Normally, to use the GPS navigation system, a user must receive signals from at least four satellites in order to solve the variables of longitude, latitude, altitude and time that are needed to precisely determine location.
Each GPS satellite transmits a radio signal at two carriers. The frequency of the two carriers are 1.57542 GHz and 1.2276 GHz, respectively. The 1.57542 GHz carrier is herein refereed to as the L1 carrier and the 1.2276 Ghz carrier is refereed to as the L2 carrier. Both carrier signals are binary phase modulated with a 50 bits per second (bps) navigation data message that provides the satellite orbital information and other information. In addition, the L1 carrier is further modulated with a 1.023 mega-chips per second (Mcps) coarse acquisition spectrum-spreading code sequence, (herein C/A-code), and a 10.23 Mcps precision code sequence or its encrypted version, (herein P(Y)-code).
The L2 carrier is modulated, as it stands today, only with the 10.23 Mcps P(Y)-code. A C/A code sequence is 1 ms long while a P(Y)-code has a periodicity of exactly one week. Each spectrum-spreading code sequence is unique for a satellite and is used as the identifier for that satellite. Since GPS satellites are equipped with and controlled by a set of ultra precise atomic oscillators, the GPS signal carrier phase and its modulating codes are exactly the reading of the onboard atomic clock""s GPS time at which the signal is transmitted.
When travelling from a GPS satellite to a near earth surface user, the GPS signal experiences a propagation delay about 76 ms. The length of the propagation delay directly relates to the distance range between the GPS satellite and the user. Due to the relative motion between the transmitting satellite and the user, the GPS signal is either stretched or squeezed at reception, as compared to its original form at transmission. This is the so-called Doppler effect and the additional frequency incurred to the signal is the Doppler frequency shift.
A GPS receiver attempts to measure the propagation delay (range) and Doppler frequency shift (range rate) in the received signal and to demodulate the navigation data bits. Since the transmission time is embedded in the signal carrier and code phase, a locally generated replica is used to match or correlate with the incoming signal in both time and frequency in order to provide the satellite clock reading of the transmission. Each GPS receiver has its own clock, albeit inexpensive, and uses it to mark the signal at reception relative to this local time base. The difference between the two time tags is a measurement of the respective signal propagation delays and the range to each satellite is then calculated by multiplying each delay by the speed of light.
A successful correlation between the local replica and the incoming signal removes the spectrum-spreading code and identifies which GPS satellite is being received. This despreading process increases the signal to noise ratio (SNR) and allows the only-remaining navigation data bits to be demodulated. The navigation data bits provide the precise orbital location of the satellite and other error-correcting coefficients. The location and time of the user are then found by solving known equations that incorporate the measured range to the known location of the GPS satellites.
A typical GPS receiver consist of four subsystems. Those subsystems include a radio frequency (RF) front-end with a pre-amplified antenna, a baseband processor, a navigation processor, and a user interface. The RF front-end down converts the GPS signal from the L1 carrier at a GHz range to a suitable intermediate frequency (IF) at a MHz range before sampling and quantization. A conventional baseband processor closes typically twelve identical tracking channels, which contain mixers, accumulate and dump (i.e., early, punctual, late correlators), numerical controlled oscillators (NCOs), a code generator. The input from the RF front-end to the baseband processor is at the MHz range but the software closure of tracking channels by the baseband processor after correlators operates at the kHz range. In addition to code and carrier tracking loops closure, the baseband processor also conducts navigation data demodulation and GPS observable generation at appropriate rates. Finally, the navigation processor operates at the several Hz range for GPS satellite orbit calculation, navigation solution, and data input/output to the user.
Since the inception, GPS receiver technologies have made steady transitions from analog to digital, from single-channel sequential-multiplexing to multiple-channel parallelism, and from GPS-alone to an integrated GPS/GLONASS (Global Navigation Satellite Systemxe2x80x94the Russian equivalent of GPS). Other advancements include the use of narrowly-spaced correlators for multipath reduction, massively parallel correlators for direct P(Y)-code acquisition, adaptive null-forming antennas for jamming suppression, and high-precision differential static and kinematic carrier phase positioning. However, these progresses are made mostly for conventional GPS receivers.
Conventional GPS receivers share a common architecture composed of two separate local loops. The local loops include a navigation loop and both a delay-locked loop and a phase/frequency-locked loop. The delay-locked loop and phase/frequency-locked loop are the physical tracking channels closed by software which are each assigned to a particular satellite. Code phase and carrier phase/frequency measurements are taken from the tracking channels, from which pseudorange, delta range, and integrated beat carrier phase observables are generated. These raw observables are then handed over to the navigation loop in which a Kalman filter or a least-squares estimator is implemented to produce position fixes.
Despite of popular use, it has been recognized that the conventional receiver architecture is inherent of two technical problems. One is the bandwidth tradeoff between noise performance and dynamic responsiveness. External aiding has been sought as a remedy to this problem but only achieved a mixed success. Moreover, the tracking loops, designed based upon small-error linear models, may be affected adversely by the actual nonlinearity effort of error discriminants as well as signal power variations under large jerk and strong interference. The other problem is the coupling between the two local loops. Ideally, all local loops of the tracking channels ought to be independent from one another. Their measurements are then recombined in the navigation loop of a Kalman filter. The Kalman filter only expects white noise in the other loop measurements. In practice, however, the loop measurements sent to the Kalman filter are neither xe2x80x9cindependentxe2x80x9d nor xe2x80x9cwhitexe2x80x9d. This is because the bandwidth of the local tracking loops is not infinite and that most receivers implement at least a point-solution to estimate the user clock bias and drift. Although some techniques have been proposed to take advantage of the over-determined inter-channel statistical correlation, they still utilize the conventional architecture and thus still face an uneasy task to move from an initial acquisition mode into the desired coupled-tracking mode.
To overcome the difficulties encountered by conventional designs, several techniques have been proposed. However, the techniques heretofore known either offer a partial and incomplete solution or a solution only workable under special settings. Such prior art techniques are exemplified by U.S. Pat. No. 4,601,005 to John Kilvington (1986), U.S. Pat. No. 4,701,934 to Steve Jasper (1987), U.S. Pat. No. 4,785,463 to Jane et al. (1988), and U.S. Pat. No. 4,998,111 to Stephen Ma et al. (1991). In such prior art systems, some suffer from excess computation. Others provide no concrete means to support critical yet no-trivial tasks such as measurement generation and offer no better way than throwing away those samples including a data bit sign reversal. This reduces the effective length of integration and the associated signal to noise ratio.
A need therefore exits for a novel GPS software receiver architecture and enabling transform-domain techniques that extend the operating envelope to high-dynamic conditions and interference-rich environment. This need is met by the present invention as described and claimed below.
The present invention is a GPS receiver and associated method that embodies high-dynamic, fast acquisition, and interference-suppressing capabilities for the reception and processing of GPS signals from a plurality of GPS satellites to produce GPS signal time and frequency parameters and navigation fixes. The GPS receiver includes an antenna and an analog front-end to intercept the incoming radio-frequency signal, band-limiting amplify the signal, and to convert it to an appropriate intermediate frequency so that it may be converted to digital form. One or more high-speed digital signal processors (DSP) constitute an all digital software baseband processor that process the sampled and quantified signals to form a two-dimensional delay-Doppler map of correlation power and to extract the signal time and frequency parameters and navigation data. The baseband processor is organized into functionally identical channels, each dynamically assigned to a different satellite visible. The baseband processor performs the incoming signal time-tagging, transformation, replica generation, interference-suppressing, and delay-Doppler mapping. The baseband processor also performs integration for larger processing gain, parameter extraction, and operational management. A relatively slow-speed microprocessor, coupled to the baseband processor, integrates the signal time and frequency parameters and navigation data from a plurality of GPS satellites to produce a navigation solution by a Kalman filter or a least-squares estimator.